This would be a very simple adaptation to make to the Online Users' block. It just means detecting if a user has filled in the Skype ID on their profile and then adding the Skype Javascript button embedder like this: https://dev.skype.com/skype-uri/generator

You'll need to make a copy of the Online Users (moodle/blocks/online_users/) block module and rename it and the class names to something else, then put in the necessary JS and PHP code to make it show the users' Skype buttons.

If you're not sure how to do it, contact your IT support or ask a developer. It's a quick, simple job.